The present invention relates to a thermally conductive element placed between a heat source and a heat sink, and more particularily to a heat conducting coiled spring for cooling electronic components in high density microelectronic packaging applications in which components are mounted on interconnect substrates, heat sinks are positioned in close proximity to the components, and springs laid on their sides are positioned in contact therebetween.
The need to remove heat from densely packed electronic components to prevent overheating is exacerbated by the trend towards higher component density and higher power dissipation. For instance, cooling from the backside of chips that are "flip-chip" or "flip-TAB" bonded to the substrate is usually necessary for packages on thick multilayer boards due to the low thermal conductivity of the electrical interconnect between the chip and the board. For such cooling systems, it becomes necessary to provide a good thermal link between a component and a heat sink such as a cooling plate. Thermal grease has been used for this purpose with the disadvantages that the grease can migrate or dry up with age, resulting in cracks that interfere with heat transfer. Thermal grease problems also arise from air pockets and nonuniform spacing among the heat sources and heat sink.
Suitable liquid coolants have been used for submersing electronic components but this can result in corrosion of both the component and the metallurgy on the substrate.
Cooling pistons have also been used but suffer the drawbacks that interface contact becomes difficult with tilted components, conformal means are usually necessary at the piston end, the components may be damaged by inertial shocks, and even with good contact the thermal efficiency is low.
Another solution used to improve thermal contact is by applying more pressure to force the surfaces together, for instance 100 pounds per module. This is limited by the pressure each individual chip can withstand, which is commonly approximately 1.5 lbs./chip. This also requires an exceptionally rigid heat sink as well as a mechanism to apply and release the pressure so that the components can be maintained or replaced.
It is also known to use thermally conductive particle filled elastomer materials as heat conduction elements. These materials can be thin films cut to fit over the components; however, they require high pressure to compensate for surface irregularities or for nonuniform spacing and therefore require excessive force for multi-chip modules.
The use of solder to improve a thermal contact has also been used. The major disadvantage of hard solders is their rigidity and high cost because of the gold contact Soft solder use is commonplace in semiconductor packaging due to its ease of use and low cost. However, the useful life of soft solder is limited by metallurgical fatigue from propogation of cracks through the solder due to the large deformation that usually occurs whenever the component is turned on and off. In addition, metallurgical bonds make repair and rework of a multi-chip module difficult.
Spring-like elements for thermal conduction between a heat source and a heat sink are also well known. Usually the spring elements provide a compliant contact, but the generally limited contact area often results in high thermal resistance. The use of solder to improve this contact can make the spring member highly rigid. For example, IBM Technical Disclosure Bulletin, Vol. 28, No. 12, pp. 5504-5505 (May, 1986) describes radial finger cooling (RFC) using springs connected at one end to an integrated circuit chip and at the other end to a cap. The Bulletin further indicates RFC does not permit a low enough thermal resistance for advanced future products. U.S. Pat. No. 4,465,130 discloses a wire form heat exchanger element with helical wire wrapped around a metallic retainer plate. This heat exchanger has relatively low compliance, high thermal resistance due to the long thermal conduction path, and limited contact at the surfaces since contact is made along the length of the wire to a flat surface. U.S. Pat. No. 4,689,720 discloses a thermal link via a resilient strip of heat conductive material, but suffers the drawbacks of relatively low contact density and a complex form. The '720 also discloses a thermal link by means of an elastomeric material filled with particles, metal wire, or wire meshes. Elastomers tend to have low thermal conductivities. Additionally, their thermal conductance can decrease after a load is applied since elastomeric materials tend to creep and relax the contact pressure. Finally, U.S. Pat. No. 4,479,140 discloses a thermal conduction bridge element with an axially compressible spring with a bulged central portion. The leaf springs used therein have relatively long thermal conductions paths which tend to lower the thermal conductance.